If $x \oplus y = 2x-6$ and $x \bigtriangleup y = xy+2x-y$, find $(-1 \bigtriangleup -3) \oplus 0$.
Answer: First, find $-1 \bigtriangleup -3$ $ -1 \bigtriangleup -3 = -(-3)+(2)(-1)-(-3)$ $ \hphantom{-1 \bigtriangleup -3} = 4$ Now, find $4 \oplus 0$ $ 4 \oplus 0 = (2)(4)-6$ $ \hphantom{4 \oplus 0} = 2$.